photoelectro-Fenton Process Research Articles

Degradation of the insecticide propoxur by electrochemical advanced oxidation processes using a boron-doped diamond/air-diffusion cell.

A solution with 0.38mM of the pesticide propoxur (PX) at pH 3.0 has been comparatively treated by electrochemical oxidation with electrogenerated H2O2 (EO-H2O2), electro-Fenton (EF), and photoelectro-Fenton (PEF). The trials were carried out with a 100-mL boron-doped diamond (BDD)/air-diffusion cell. The EO-H2O2 process had the lowest oxidation ability due to the slow reaction of intermediates with •OH produced from water discharge at the BDD anode. The EF treatment yielded quicker mineralization due to the additional •OH formed between added Fe2+ and electrogenerated H2O2. The PEF process was the most powerful since it led to total mineralization by the combined oxidative action of hydroxyl radicals and UVA irradiation. The PX decay agreed with a pseudo-first-order kinetics in EO-H2O2, whereas in EF and PEF, it obeyed a much faster pseudo-first-order kinetics followed by a much slower one, which are related to the oxidation of its Fe(II) and Fe(III) complexes, respectively. EO-H2O2 showed similar oxidation ability within the pH range 3.0-9.0. The effect of current density and Fe2+ and substrate contents on the performance of the EF process was examined. Two primary aromatic products were identified by LC-MS during PX degradation.

Advanced oxidation treatment of malachite green dye using a low cost carbon-felt air-diffusion cathode

A low cost carbon-felt (CF) material has been successfully employed as an air-diffusion cathode for the generation of H2O2 from O2 reduction in 0.05M Na2SO4 at pH 3. Using a BDD (Boron Doped Diamond)/CF air-diffusion electrode (3cm2), a maximum of 94mgL−1 of H2O2 were accumulated for the best current density value identified (21.7mAcm−2). These results were technically and economically better than other performances observed for similar cathode materials in O2-saturated solutions. An electrochemical reactor utilizing these electrodes was employed to degrade malachite green (MG) dye in the same electrolyte containing 0.5mM of Fe2+ using the electro-Fenton (EF) and photoelectro-Fenton (PEF) processes. The main oxidant was HO formed at the BDD surface from water oxidation and in the bulk from Fenton’s reaction between H2O2 and added Fe2+. As expected, the PEF process was more powerful than the EF due to the enhancement of HO production by photolysis of Fe(III)-hydroxy species and of Fe(III) complexes of the oxalate of the dye, as well as of intermediates. The effect of MG concentration on decolorization and mineralization rates, mineralization current efficiency and energy consumption for both processes were assessed. The CF air-diffusion cathode lost progressively performance upon consecutive PEF treatment of a 150mgL−1 MG owed to the gradual deposition of Fe species, as well as an enrichment of O, Na and S species on its surface. The deposition of Fe species was reversible since these were dissolved in the medium in consecutive trials without the need of adding new amounts of Fe2+.

Mineralization of the textile dye acid yellow 42 by solar photoelectro-Fenton in a lab-pilot plant

A complete mineralization of a textile dye widely used in the Chilean textile industry, acid yellow 42 (AY42), was studied. Degradation was carried out in an aqueous solution containing 100mgL−1 of total organic carbon (TOC) of dye using the advanced solar photoelectro-Fenton (SPEF) process in a lab-scale pilot plant consisting of a filter press cell, which contains a boron doped diamond electrode and an air diffusion cathode (BDD/air-diffusion cell), coupled with a solar photoreactor for treat 8L of wastewater during 270min of electrolysis.The main results obtained during the degradation of the textile dye were that a complete transformation to CO2 depends directly on the applied current density, the concentration of Fe2+ used as catalyst, and the solar radiation intensity. The elimination of AY42 and its organic intermediates was due to hydroxyl radicals formed at the anode surface from water oxidation and in the bulk from Fenton’s reaction between electrogenerated H2O2 and added Fe2+. The application of solar radiation in the process (SPEF) yield higher current efficiencies and lower energy consumptions than electro-Fenton (EF) and electro-oxidation with electrogenerated H2O2 (E OH2O2) by the additional production of hydroxyl radicals from the photolysis of Fe(III) hydrated species and the photodecomposition of Fe(III) complexes with organic intermediates.Moreover, some products and intermediates formed during mineralization of dye, such as inorganic ions, carboxylic acids and aromatic compounds were determined by photometric and chromatographic methods. An oxidation pathway is proposed for the complete conversion to CO2.

Use of a carbon felt–iron oxide air-diffusion cathode for the mineralization of Malachite Green dye by heterogeneous electro-Fenton and UVA photoelectro-Fenton processes

A novel carbon felt cathode coated with iron oxides has been prepared by Fe3+ electrodeposition. The deposited iron content was analyzed by X-ray fluorescence, the composition of iron oxides formed by X-ray diffraction and the morphology of deposits by scanning electron microscopy and energy-dispersive X-ray spectroscopy. Freshly prepared electrodes were used as air-diffusion cathodes in a stirred tank reactor with a boron-doped diamond (BDD) anode to degrade solutions of Malachite Green dye at pH3.0 by heterogeneous electro-Fenton (EF) and photoelectro-Fenton (PEF) processes. Oxidizing agents were hydroxyl radicals formed at the BDD surface from water oxidation and at the cathode surface or in the bulk from Fenton's reaction between deposited or leached Fe2+ and electrogenerated H2O2 in both treatments, along with the photo-oxidation by UVA light in heterogeneous PEF. The decolorization and mineralization of dye solutions were faster for the latter process due to the photolysis of Fe(III) complexes with initial oxalate and generated carboxylic acids. TOC decay always obeyed a pseudo-first-order kinetics. The increase in dye concentration decelerated the decolorization efficiency and mineralization rate, but enhanced the mineralization current efficiency. A test of 10 consecutive cycles of heterogeneous EF treatment of 50mgL−1 of Malachite Green at 21.7mAcm−2 showed a gradual loss of iron oxides from the cathode with decreasing dye mineralization rate. An improvement of the stability of the novel cathode is then necessary for its reuse with a better keeping of the oxidation power of heterogeneous EF and PEF treatments of organic pollutants in waters.

Effective removal of Orange-G azo dye from water by electro-Fenton and photoelectro-Fenton processes using a boron-doped diamond anode

Abstract Coupled electrochemical advanced oxidation processes like electro-Fenton (EF) and photoelectro-Fenton (PEF) were evaluated for the treatment of an aqueous solution containing 100 mg L –1 total organic carbon of the low biodegradable azo dye Orange-G (Acid Orange 10) using a boron-doped diamond (BDD)/air-diffusion cell. It was confirmed the existence of synergic effects between UVA light photo-oxidation and/or hydroxyl radicals ( OH) formed from water oxidation at the BDD anode and the Fenton reaction between added Fe 2+ and H 2 O 2 produced at the air-diffusion cathode. A rapid discoloration of the solution was always obtained, mainly attributed to the oxidation of the azo dye with OH generated from Fenton’s reaction in EF and PEF. The color loss followed a pseudo-first-order kinetics, controlled by the applied current. The dye mineralization was enhanced with increasing current due to the greater production of OH and finally, short-linear carboxylic acids like oxalic and oxamic were pre-eminently accumulated. The Fe(III) complexes of these acids were slowly removed by OH in EF and rapidly photodecomposed by UVA light in PEF up to 98% mineralization. Sulfate and nitrate ions were accumulated in the medium during both EF and PEF treatments. The more powerful coupled PEF process is then able to efficiently degrade streams contaminated with Orange G.

Comparison of Photo ElectroFenton Process(PEF) and combination of PEF Process and Membrane Bioreactor in the treatment of Landfill Leachate

Landfill leachate treatment is an integral part of municipal solid waste management. The conventional biological treatment of landfill leachate is limited because of the presence of toxic contaminants and recalcitrant organics. Among Advanced Oxidation processes(AOPs), Photo Electro Fenton Process(PEF) can be used effectively for the treatment of highly contaminated water. In the treatment of leachate by biological methods, Membrane Bioreactor(MBR) is efficient while considering the effluent quality. So a combination of PEF followed by MBR treatment is adopted in this study. The percentage removal of pollutants -TSS, BOD,COD, Ammonia Nitrogen, Phosphate, Sulphate, Sulphide and Chloride from landfill leachate after PEF process is 89.3,71.9,83.6,100,58,92.3,65 and 65 respectively. The percentage removal of the same pollutant parameters after combined treatment of PEF followed by MBR is 95.5,90.2,96.2,100,82.7,93.3,88.2 and 88.3. The pollutant removal efficiency is increased by adopting combined treatment –PEF followed by MBR.

Up to 95 % reduction of chemical oxygen demand of slaughterhouse effluents using Fenton and photo-Fenton oxidation

Meat industries produce effluents containing high concentrations of organic and inorganic compounds, which must be removed before being discharged or reused. Advanced oxidation processes using Fenton reaction coupled with UV, solar radiation, and electrochemical oxidation are promising methods. Here, we treated the effluent from an anaerobic digester using: (a) the photoelectro-Fenton process, using a system with a Ti-RuO2 anode and a carbon felt cathode, (b) the solar photo-Fenton process, using a batch reactor and a compound parabolic collector, and (c) a combination of Fenton and solar photo-Fenton processes. The effluent had an initial chemical oxygen demand (COD) of 1159 mgL−1, and we obtained high removal efficiencies of COD, up to 95 %, using the combination of Fenton and solar photo-Fenton processes.

Optimization of two-chamber photo electro Fenton reactor for the treatment of winery wastewater

The treatment of winery wastewater is necessary since it constitutes an environmental problem due to its high organic content and chemical oxygen demand and its low pH. However, these characteristics hinder the use of conventional technologies commonly utilized for the treatment of effluents. Therefore, new technologies for the management of this type of wastewater are required. In this sense, the photo electro Fenton process (PEF) was proposed as a good alternative because of the synergetic effect among Fenton, electrolysis and photolysis processes. In this study, the development of a new double chamber cubic reactor for the treatment of winery wastewater using PEF was performed. Surface response methodology was applied based on Box–Behnken design to define the best operational conditions. The selected key variables were voltage, distance between electrodes and the organic load of the effluent. Among the parameters optimized, distance between electrodes and voltage were identified as significant in the model. Under the optimized conditions the treatment of real winery wastewater was efficiently carried out. Finally, it can be concluded that the configuration of this reactor is suitable for the remediation of this type of effluents.

Routes for the electrochemical degradation of the artificial food azo-colour Ponceau 4R by advanced oxidation processes

The performance of three electrochemical advanced oxidation processes, namely electro-oxidation with electrogenerated H2O2 (EO–H2O2), electro-Fenton (EF) and photoelectro-Fenton (PEF) for the treatment of aqueous solutions of the food azo dye Ponceau 4R in an undivided cell with a BDD anode and an air-diffusion cathode was compared in terms of colour, dye concentration and total organic carbon (TOC) removals. PEF treatments in ultrapure water with Na2SO4 were performed to assess the effect of current density, as well as supporting electrolyte and dye concentrations. At 100mAcm−2, solutions of 130mL of 254mgL−1 of the dye in 0.05M Na2SO4 became colourless and totally mineralized after 50 and 240min, respectively, which can be explained by the synergistic action of BDD(OH) at the anode surface and homogeneous OH formed in the bulk from Fenton’s reaction promoted in the presence of Fe2+ catalyst. Furthermore, UVA photons induced the continuous Fe2+ regeneration and photolytic decomposition of refractory intermediate complexes. In that aqueous matrix, the cleavage of the dye molecules proceeded through several reaction routes to yield N-containing and non-N-containing derivatives with one or two aromatic rings, short-chain aliphatic carboxylic acids and inorganic ions. Oxalic and oxamic acids and sulfate ions were accumulated at different rates in EO–H2O2, EF and PEF. The three methods allowed the progressive decontamination of Ponceau 4R solutions in a real water matrix even without the addition of electrolyte, although complete TOC abatement after 360min at 33.3mAcm−2 was only ensured by the iron-catalyzed PEF process.

Incorporation of electrochemical advanced oxidation processes in a multistage treatment system for sanitary landfill leachate

The current study has proved the technical feasibility of including electrochemical advanced oxidation processes (EAOPs) in a multistage strategy for the remediation of a sanitary landfill leachate that embraced: (i) first biological treatment to remove the biodegradable organic fraction, oxidize ammonium and reduce alkalinity, (ii) coagulation of the bio-treated leachate to precipitate humic acids and particles, followed by separation of the clarified effluent, and (iii) oxidation of the resulting effluent by an EAOP to degrade the recalcitrant organic matter and increase its biodegradability so that a second biological process for removal of biodegradable organics and nitrogen content could be applied. The influence of current density on an UVA photoelectro-Fenton (PEF) process was firstly assessed. The oxidation ability of various EAOPs such as electro-Fenton (EF) with two distinct initial total dissolved iron concentrations ([TDI]0), PEF and solar PEF (SPEF) was further evaluated and these processes were compared with their analogous chemical ones. A detailed assessment of the two first treatment stages was made and the biodegradability enhancement during the SPEF process was determined by a Zahn–Wellens test to define the ideal organics oxidation state to stop the EAOP and apply the second biological treatment. The best current density was 200 mA cm−2 for a PEF process using a BDD anode, [TDI]0 of 60 mg L−1, pH 2.8 and 20 °C. The relative oxidation ability of EAOPs increased in the order EF with 12 mg [TDI]0 L−1 < EF with 60 mg [TDI]0 L−1 < PEF with 60 mg [TDI]0 L−1 ≤ SPEF with 60 mg [TDI]0 L−1, using the abovementioned conditions. While EF process was much superior to the Fenton one, the superiority of PEF over photo-Fenton was less evident and SPEF attained similar degradation to solar photo-Fenton. To provide a final dissolved organic carbon (DOC) of 163 mg L−1 to fulfill the discharge limits into the environment after a second biological process, 6.2 kJ L−1 UV energy and 36 kWh m−3 electrical energy were consumed using SPEF with a BDD anode at 200 mA cm−2, 60 mg [TDI]0 L−1, pH 2.8 and 20 °C.

Photoelectrocatalytic/photoelectro-Fenton coupling system using a nanostructured photoanode for the oxidation of a textile dye: Kinetics study and oxidation pathway

In this study, a coupled photoelectrocatalytic/photoelectro-Fenton reactor was designed to enhance the degradation efficiency of organic pollutants and tested using the azo dye Orange G as a model compound. Pt-decorated TiO2 nanotubes were used as a photoanode with an air-diffusion polytetrafluoroethylene cathode for H2O2 generation. The sum of individual effects of coupling the photoelectrocatalytic and photoelectro-Fenton processes was evaluated as a function of the decolorization and mineralization of Orange G solutions. The dye solutions were only completely decolorized in more acidic conditions (pH 3.0). The mineralization of the Orange G solutions increased in the sequence photoelectrocatalytic<electro-Fenton<coupled photoelectrocatalytic/photoelectro-Fenton due to the gradual increase in the production of OH radicals. Total organic carbon reductions of 80% for photoelectrocatalysis, 87% for electro-Fenton and 97% for the coupled processes were obtained when using an applied electric charge per unit volume of electrolyzed solution of 200mAhL−1. The Orange G decays for all treatments followed pseudo-first-order kinetics, suggesting the attack of a constant concentration of OH radicals. Aromatics such as naphthalenic and benzenic compounds were formed as by-products and were identified using LC–MS/MS analysis. In addition, the generated aliphatic acids were identified using ion-exclusion high-performance liquid chromatography. The final by-products of oxalic and formic acid were identified as ultimate by-products and formed Fe(III) complexes that were rapidly mineralized to CO2 by UV–Vis irradiation. Then, according to the identified oxidation by-products, a plausible pathway was proposed for the degradation of Orange G dye by the coupled process.

Electrochemical and photo-assisted electrochemical treatment of the pesticide imidacloprid in aqueous solution by the Fenton process: effect of operational parameters

The aim of this study was to compare the degradation efficiency (DE%) of imidacloprid as a model pesticide by electro-Fenton (EF) and photoelectro-Fenton processes (PEF) using undivided three-electrode electrochemical cell and UV irradiation in a batch mode. The potential of the working electrode (graphite) was fixed at −1.0 V versus the saturated calomel electrode. The selected operating conditions for treatment of imidacloprid (20 mg/L) were: pH 2.8, Fe2+ concentration of 0.36 mM and Na2SO4 concentration of 0.15 M as the background electrolyte, which produced a DE% of 59.23 and 80.49 % for EF and PEF after 180 min, respectively. Considerable synergistic effect between EF and UV processes was observed due to the regeneration of Fe2+ ions and more production of hydroxyl radicals (·OH). Besides, accumulation of the electro-generated H2O2 in the electrochemical system as the source of ·OH radicals was confirmed. Moreover, total organic carbon measurements under the optimized condition demonstrated that 50.73 and 67.15 % of the organic substrates were mineralized after 300 min of the treatment by the EF and PEF, respectively. Eventually, the experimental results revealed that the degradation and mineralization rates of the pesticide followed pseudo-first-order kinetics; however, the rate constants of the mineralization were lower than the degradation ones owing to the generated intermediates, which required more treatment during the processes.

Comparison of Processes for the Degradation of Trans-Cinnamic Acid: Anodic Oxidation, Electro-Fenton and Photoelectro-Fenton

INTRODUCTION Olive mill wastes worsen environmental problems in Mediterranean countries. These wastewaters contain phenolic compounds used as fragrances and artificial flavours and other compounds such as trans-cinnamic acid. The electrochemical techniques, especially the electrochemical advanced oxidation processes (EAOPs), are applied in many fields including the wastewater treatment. EAOPs are based on the in situ electrochemical generation of the hydroxyl radical (●OH), which has so high oxidizing power that it is able to yield dehydrogenated or hydroxilated derivatives that can be oxidized up to their complete mineralization. EXPERIMENTAL In this work, the degradation of trans-cinnamic acid was studied by means of various EAOPs. Solutions of 100 mL of this compound in 0.05 M Na2SO4 of pH 3.0 were comparatively treated by anodic oxidation with electrogenerated H2O2 (AO-H2O2), electro-Fenton (EF) and photoelectro-Fenton (PEF) using a stirred tank reactor equipped with a 3 cm2 boron-doped diamond (BDD) anode and a 3 cm2 carbon-polytetrafluoroethylene air-diffusion cathode. In AO-H2O2, the acid and its intermediates were destroyed by ●OH formed at the BDD surface, whereas in EF the mineralization rate was enhanced by ●OH generated in the bulk from Fenton’s reaction between cathodically generated H2O2 and 0.5 mM Fe2+ added at the beginning of the experiment. In the case of PEF, the degradation process can be significantly enhanced under the photocatalytic action of UVA radiation provided by a 6 W artificial lamp. For each method, the influence of current density and trans-cinnamic acid content on TOC decay was examined. The results revealed that the PEF process was always the most efficient treatment, allowing an almost total mineralization of a 100 mg L-1 TOC solution of trans-cinnamic acid after 180 min of electrolysis at 33.3 mA cm-2. Generated short-chain aliphatic carboxylic acids as oxalic, acetic and fumaric in the above treatments were identified and quantified by ion-exclusion HPLC. RESULTS For all the EAOPs tested, TOC was gradually reduced at prolonged electrolysis. The oxidation ability increased in the sequence AO-H2O2 < EF < PEF. For each method, the mineralization was enhanced with increasing current density. PEF became the most potent method, yielding 98% mineralization after 180 min at 33.3 and 66.7 mA cm-2, which showed that UVA light was a powerful tool. TOC removal increased at a higher initial trans-cinnamic acid concentration, especially in the EF and PEF assays, thereby making the process much more efficient. HPLC chromatograms of electrolyzed solutions exhibited well-defined peaks related to oxalic (t r = 6.96 min), acetic (t r = 15.1 min) and fumaric (t r = 15.6 min) acids. The two latter acids are formed from the cleavage of the benzene ring of trans-cinnamic acid, being oxidized to oxalic acid that is directly and completely mineralized to CO2in 240 min by PEF. DISCUSSION Trans-cinnamic acid solutions can be decontaminated at pH 3 and 25 °C using a BDD/air-diffusion cell. This is due to the oxidation with ●OH formed from water oxidation at the BDD surface in AO-H2O2, the additional destruction with ●OH produced in the bulk from Fenton’s reaction between generated H2O2 and added Fe2+in EF, and the synergistic action of photolysis of Fe(III)-carboxylate complexes under UVA irradiation in PEF. The mineralization rate of trans-cinnamic acid increases in the order AO-H2O2 < EF < PEF under the same experimental conditions due to the larger production of ●OH and the action of UVA light. For each EAOP, higher mineralization rate was found at higher current density as a result of the higher generation of ●OH at the BDD surface and in the bulk, but the mineralization current efficiency decreased because of the enhancement of parasite reaction of ●OH. An increase in trans-cinnamic acid concentration caused a higher TOC removal with superior current efficiency due to the minimization of parasite reactions of ●OH by the presence of more organic matter. The destruction of the parent molecule and its intermediates yielded final short-chain carboxylic acids such as fumaric, acetic and oxalic. The latter was the most persistent but its Fe(III) complexes could be rapidly photodecomposed in PEF. The PEF process was the most potent EAOP giving rise to an almost total mineralization of solutions with 100 mg L-1 TOC of trans-cinnamic acid after 180 min at 33.3 mA cm-2. The EAOPs tested demonstrated their feasibility to decontaminate water containing trans-cinnamic acid, opening the door to develop more effective technologies for the treatment of olive mill wastewaters. ACKNOWLEDGEMENTS The authors are grateful to MINECO (Spain) for financial support from project CTQ2013-48897-C2-1-R, co-financed with FEDER funds. The scholarship given to N.E Flores from SENECYT (Ecuador) is also acknowledged.

Electro-Fenton and solar photoelectro-Fenton treatments of the pharmaceutical ranitidine in pre-pilot flow plant scale

Here, 2.5L of solutions of the pharmaceutical ranitidine (RNTD) at pH 3.0 have been comparatively degraded by electro-Fenton (EF) and solar photoelectro-Fenton (SPEF) processes using a pre-pilot flow plant with a Pt/air-diffusion cell. RNTD was oxidized by hydroxyl radicals coming from water oxidation at the Pt anode and Fenton’s reaction between added Fe2+ and H2O2 generated at the air-diffusion cathode. In SPEF, the cell was coupled to a flat solar photoreactor to irradiate the solution with sunlight. The potent combined action of hydroxyl radicals and photolysis by sunlight explains the higher oxidation ability of SPEF compared to EF, attaining 80% mineralization as maximal. The optimum Fe2+ content as catalyst was 0.50mM. The effect of current density and drug concentration on the degradation rate of RNTD, mineralization current efficiency and energy consumption for EF and SPEF was examined. The RNTD decay always followed a pseudo-first-order kinetics, having a greater oxidation rate for SPEF by the additional generation of hydroxyl radicals induced by photolysis of Fe(III) species. Malic, pyruvic, acetic, oxalic, oxamic and formic acids were detected as final carboxylic acids. The three latter acids were the main components of the final treated solution in EF because their Fe(III) complexes were not destroyed by generated hydroxyl radicals. The quick photolysis of such Fe(III)-carboxylate species by sunlight explains the greater oxidation power of SPEF. The initial S atom of RNTD was released as sulfate ion, whereas its initial N atoms were converted into ammonium ion along with a smaller proportion of nitrate ion.

H2O2-assisted TiO2 generation during the photoelectrocatalytic process to decompose the acid green textile dye by Fenton reaction

Abstract TiO 2 electrode was used as photo-driven anode for oxygen reduction at carbon-felt cathode to produce H 2 O 2 used in the Fenton reaction to decompose an acid green 50 textile dye (AG50). The H 2 O 2 generated was catalyzed with ferrous ions in the photoelectroFenton process to decompose AG50 in cathode compartment. TiO 2 nanocomposite electrode was prepared by electrochemical anodization of pure Ti substrates. The results showed that TiO 2 anode exhibited a porous surface and organized nanotubes annealed at 450 °C and 600 °C with anatase phase, while a surface with less porous density with compact bars was obtained at 700 °C with anatase/rutile phase. The influence of applied bias potential, concentration of methanol in the anolyte, annealing temperature of TiO 2 nanocomposite and pH of the anolyte were studied on the photoelectrocatalytic degradation of methanol, whereas the catholyte composition remained unchanged for H 2 O 2 electrogeneration. Higher mineralization of both organic compounds was obtained by increasing the annealing temperature under acidic conditions using a bias potential of 1.02 V.

Remediation of a winery wastewater combining aerobic biological oxidation and electrochemical advanced oxidation processes

Apart from a high biodegradable fraction consisting of organic acids, sugars and alcohols, winery wastewaters exhibit a recalcitrant fraction containing high-molecular-weight compounds as polyphenols, tannins and lignins. In this context, a winery wastewater was firstly subjected to a biological oxidation to mineralize the biodegradable fraction and afterwards an electrochemical advanced oxidation process (EAOP) was applied in order to mineralize the refractory molecules or transform them into simpler ones that can be further biodegraded. The biological oxidation led to above 97% removals of dissolved organic carbon (DOC), chemical oxygen demand (COD) and 5-day biochemical oxygen demand (BOD5), but was inefficient on the degradation of a bioresistant fraction corresponding to 130 mg L−1 of DOC, 380 mg O2 L−1 of COD and 8.2 mg caffeic acid equivalent L−1 of total dissolved polyphenols. Various EAOPs such as anodic oxidation with electrogenerated H2O2 (AO-H2O2), electro-Fenton (EF), UVA photoelectro-Fenton (PEF) and solar PEF (SPEF) were then applied to the recalcitrant effluent fraction using a 2.2 L lab-scale flow plant containing an electrochemical cell equipped with a boron-doped diamond (BDD) anode and a carbon-PTFE air-diffusion cathode and coupled to a photoreactor with compound parabolic collectors (CPCs). The influence of initial Fe2+ concentration and current density on the PEF process was evaluated. The relative oxidative ability of EAOPs increased in the order AO-H2O2 < EF < PEF ≤ SPEF. The SPEF process using an initial Fe2+ concentration of 35 mg L−1, current density of 25 mA cm−2, pH of 2.8 and 25 °C reached removals of 86% on DOC and 68% on COD after 240 min, regarding the biologically treated effluent, along with energy consumptions of 45 kWh (kg DOC)−1 and 5.1 kWh m−3. After this coupled treatment, color, odor, COD, BOD5, NH4+, NO3− and SO42− parameters complied with the legislation targets and, in addition, a total dissolved polyphenols content of 0.35 mg caffeic acid equivalent L−1 was found. Respirometry tests revealed low biodegradability enhancement along the SPEF process.

Decolorization and mineralization of Allura Red AC aqueous solutions by electrochemical advanced oxidation processes

The decolorization and mineralization of solutions containing 230mgL−1 of the food azo dye Allura Red AC at pH 3.0 have been studied upon treatment by electrochemical oxidation with electrogenerated H2O2 (EO-H2O2), electro-Fenton (EF) and photoelectro-Fenton (PEF). Experiments were performed with a stirred tank reactor containing a boron-doped diamond (BDD) or Pt anode and an air-diffusion cathode to generate H2O2. The main oxidants were hydroxyl radicals formed at the anode surface from water oxidation and in the bulk from Fenton’s reaction between H2O2 and added Fe2+. The oxidation ability increased in the sequence EO-H2O2<EF<PEF and faster degradation was always obtained using BDD. PEF process with BDD yielded almost total mineralization following similar trends in SO42−, ClO4− and NO3− media, whereas in Cl− medium, mineralization was inhibited by the formation of recalcitrant chloroderivatives. GC–MS analysis confirmed the cleavage of the NN bond with formation of two main aromatics in SO42− medium and three chloroaromatics in Cl− solutions. The effective oxidation of final oxalic and oxamic acids by BDD along with the photolysis of Fe(III)-oxalate species by UVA light accounted for the superiority of PEF with BDD. NH4+, NO3− and SO42− ions were released during the mineralization.

Electrochemically Prepared Iron-Modified Activated Carbon Electrodes for Their Application in Electro-Fenton and Photoelectro-Fenton Processes

The presence of Ferrous ions is essential in Electro-Fenton (EF) and Photoelectro-Fenton (PEF) processes. To avoid the use of iron salts as precursors for the homogeneous Fenton process, iron was electrochemically deposited on a high surface area activated carbon (AC) material as previously reported by Santiago et al. for Pt-C particles. In this study, a novel rotating disk-slurry electrode (RoDSE) technique is employed to electrochemically prepare bulk iron/carbon (Fe/AC) material while avoiding a constant contact of the carbon support with an electrode surface. The electrochemically prepared Fe/AC was characterized, used to prepare an electrode and electrochemical experiments were performed to examine its performance as a cathode material for Electro- and Photoelectro-Fenton processes using a typical dye aqueous solution as a model effluent. The Fe/AC material exhibited a more complete (98%) and faster degradation performance, demonstrating that the use of the novel RoDSE technique is an effective method for preparing bulk quantities of activated carbon-supported iron for heterogeneous EF and PEF applications via an electrochemical route.

Electrochemical treatment of Acid Red 1 by electro-Fenton and photoelectro-Fenton processes

Small volumes (100 mL) of acidic aqueous solutions with 30-200 mg L -1 TOC of the toxic azo dye Acid Red 1 (AR1) have been comparatively treated by various electrochemical advanced oxidation processes (EAOPs). The electrolytic system consisted of a BDD anode able to produce  OH and an air-diffusion cathode that generated H2O2, which subsequently reacted with added Fe 2+ to yield additional  OH from Fenton’s reaction. Under optimized conditions (i.e., 1.0 mM Fe 2+ , 60 mA cm -2 , pH 3.0, 35 oC), the analysis of the initial rates for decolourization and AR1 decay assuming a pseudo-first-order kinetics revealed a much higher rate constant for photoelectro-Fenton (PEF, ~ 2.7x10 -3 s -1 ) compared to electro-Fenton (EF, ~ 0.6x10 -3 s -1 ). Mineralization after 180 min was also greater in the former treatment (90 % vs 63 %). The use of UV radiation in PEF contributed to Fe(III) photoreduction as well as to photodecarboxylation of refractory intermediates, yielding a mineralization current efficiency as high as 85% during the treatment of solutions of 200 mg L -1 TOC. Primary reaction intermediates included three aromatic derivatives with the initial naphthalenic structure and four molecules only featuring benzenic rings, which were totally mineralized in PEF.

Electrochemical treatment of Acid Red 1 by electro-Fenton and photoelectro-Fenton processes

Small volumes (100 mL) of acidic aqueous solutions with 30-200 mg L-1 TOC of the toxic azo dye Acid Red 1 (AR1) have been comparatively treated by various electrochemical advanced oxidation processes (EAOPs). The electrolytic system consisted of a BDD anode able to produce ·OH and an air-diffusion cathode that generated H2O2, which subsequently reacted with added Fe2+ to yield additional ·OH from Fenton’s reaction. Under optimized conditions (i.e., 1.0 mM Fe2+, 60 mA cm-2, pH 3.0, 35 ºC), the analysis of the initial rates for decolourization and AR1 decay assuming a pseudo-first-order kinetics revealed a much higher rate constant for photoelectro-Fenton (PEF, ~ 2.7x10-3 s-1) compared to electro-Fenton (EF, ~ 0.6x10-3 s-1). Mineralization after 180 min was also greater in the former treatment (90 % vs 63 %). The use of UV radiation in PEF contributed to Fe(III) photoreduction as well as to photodecarboxylation of refractory intermediates, yielding a mineralization current efficiency as high as 85% during the treatment of solutions of 200 mg L-1 TOC. Primary reaction intermediates included three aromatic derivatives with the initial naphthalenic structure and four molecules only featuring benzenic rings, which were totally mineralized in PEF.